Human respiratory syncytial virus (RSV) outranks all other microbial pathogens as a cause of pneumonia and bronchiolitis in infants under one year of age. Virtually all children are infected by two years of age, and reinfection occurs with appreciable frequency in older children and young adults (Chanock et al., in Viral Infections of Humans, 3rd ed., A. S. Evans, ed., Plenum Press, N.Y. (1989). RSV is responsible for more than one in five pediatric hospital admissions due to respiratory tract disease, and causes an estimated 91,000 hospitalizations and 4,500 deaths yearly in the United States alone. Although most healthy adults do not have serious disease due to RSV infection, elderly patients and immunocompromised individuals often suffer severe and possibly life-threatening infections from this pathogen.
Despite decades of investigation to develop effective vaccine agents against RSV, no safe and effective vaccine has yet been achieved to prevent the severe morbidity and significant mortality associated with RSV infection. Failure to develop successful vaccines relates in part to the fact that small infants have diminished serum and secretory antibody responses to RSV antigens. Thus, these individuals suffer more severe infections from RSV, whereas cumulative immunity appears to protect older children and adults against more serious impacts of the virus. One antiviral compound, ribavarin, has shown promise in the treatment of severely infected infants, although there is no indication that it shortens the duration of hospitalization or diminishes the infant's need for supportive therapy.
The mechanisms of immunity in RSV infection have recently come into focus. Secretory antibodies appear to be most important in protecting the upper respiratory tract, whereas high levels of serum antibodies are thought to have a major role in resistance to RSV infection in the lower respiratory tract. Purified human immunoglobulin containing a high titer of neutralizing antibodies to RSV may prove useful in some instances of immunotherapeutic approaches for serious lower respiratory tract disease in infants and young children. Immune globulin preparations, however, suffer from several disadvantages, such as the possibility of transmitting blood-borne viruses and difficulty and expense in preparation and storage.
RSV-specific cytotoxic T cells, another effector arm of induced immunity, are also important in resolving an RSV infection. However, while this latter effector can be augmented by prior immunization to yield increased resistance to virus challenge, the effect is short-lived. The F and G surface glycoproteins are the two major protective antigens of RSV, and are the only two RSV proteins which have been shown to induce RSV neutralizing antibodies and long term resistance to challenge (Collins et al., Fields Virology, Fields et al. eds., 2:1313-1352. Lippincott-Raven, Philadelphia. (1996); Connors et al., J. Virol. 65(3):1634-7 (1991)). The third RSV surface protein, SH, did not induce RSV-neutralizing antibodies or significant resistance to RSV challenge.
One obstacle to development of live RSV vaccines is the difficulty in achieving an appropriate balance between attenuation and immunogenicity. Genetic stability of attenuated viruses also can be a problem. Vaccine development also is impeded by the relatively poor growth of RSV in cell culture and the instability of the virus particle. Another feature of RSV infection is that the immunity which is induced is not fully protective against subsequent infection. A number of factors probably contribute to this, including the relative inefficiency of the immune system in restricting virus infection on the luminal surface of the respiratory tract, the short-lived nature of local mucosal immunity, rapid and extensive virus replication, reduced immune responses in the young due to immunological immaturity, immunosuppression by transplacentally derived maternal serum antibodies, and certain features of the virus such as a high degree of glycosylation of the G protein. Also, as will be described below, RSV exists as two antigenic subgroups A and B, and immunity against one subgroup is of reduced effectiveness against the other.
Although RSV can reinfect multiple times during life, reinfections usually are reduced in severity due to protective immunity induced by prior infection, and thus immunoprophylaxis is feasible. A live-attenuated RSV vaccine would be administered intranasally to initiate a mild immunizing infection. This has the advantage of simplicity and safety compared to a parenteral route. It also provides direct stimulation of local respiratory tract immunity, which plays a major role in resistance to RSV. It also abrogates the immunosuppressive effects of RSV-specific maternally-derived serum antibodies, which typically are found in the very young. Also, while the parenteral administration of RSV antigens can sometimes be associated with immunopathologic complications (Murphy et al., Vaccine 8(5):497-502 (1990)), this has never been observed with a live virus.
Formalin-inactivated virus vaccine was tested against RSV in the mid-1960s, but failed to protect against RSV infection or disease, and in fact exacerbated symptoms during subsequent infection by the virus. (Kim et al., Am. J. Epidemiol., 89:422-434 (1969), Chin et al., Am J. Epidemiol., 89:449-463 (1969); Kapikian et al., Am. J. Epidemiol., 89:405-421 (1969)).
More recently, vaccine development for RSV has focused on attenuated RSV mutants. Friedewald et al., J. Amer. Med. Assoc. 204:690-694 (1968) reported a cold passaged mutant of RSV (cpRSV) which appeared to be sufficiently attenuated to be a candidate vaccine. This mutant exhibited a slight increased efficiency of growth at 26° C. compared to its wild-type (wt) parental virus, but its replication was neither temperature sensitive nor significantly cold-adapted. The cold-passaged mutant, however, was attenuated for adults. Although satisfactorily attenuated and immunogenic for infants and children who had been previously infected with RSV (i.e., seropositive individuals), the cpRSV mutant retained a low level virulence for the upper respiratory tract of seronegative infants.
Similarly, Gharpure et al., J. Virol. 3:414-421 (1969) reported the isolation of temperature sensitive RSV (tsRSV) mutants which also were promising vaccine candidates. One mutant, ts-1, was evaluated extensively in the laboratory and in volunteers. The mutant produced asymptomatic infection in adult volunteers and conferred resistance to challenge with wild-type virus 45 days after immunization. Again, while seropositive infants and children underwent asymptomatic infection, seronegative infants developed signs of rhinitis and other mild symptoms. Furthermore, instability of the ts phenotype was detected, although virus exhibiting a partial or complete loss of temperature sensitivity represented a small proportion of virus recoverable from vaccinees, and was not associated with signs of disease other than mild rhinitis.
These and other studies revealed that certain cold-passaged and temperature sensitive RSV strains were underattenuated and caused mild symptoms of disease in some vaccinees, particularly seronegative infants, while others were overattenuated and failed to replicate sufficiently to elicit a protective immune response, (Wright et al., Infect. Immun., 37:397-400 (1982)). Moreover, genetic instability of candidate vaccine mutants has resulted in loss of their temperature-sensitive phenotype, further hindering development of effective RSV vaccines. See generally, Hodes et al., Proc. Soc. Exp. Biol. Med. 145:1158-1164 (1974), McIntosh et al., Pediatr. Res. 8:689-696 (1974), and Belshe et al., J. Med. Virol., 3:101-110 (1978).
Abandoning the approach of creating suitably attenuated RSV strains through undefined biological methods such as cold-passaging, investigators tested subunit vaccine candidates using purified RSV envelope glycoproteins. The glycoproteins induced resistance to RS virus infection in the lungs of cotton rats, Walsh et al., J. Infect. Dis. 155:1198-1204 (1987), but the antibodies had very weak neutralizing activity and immunization of rodents with purified subunit vaccine led to disease potentiation (Murphy et al., Vaccine 8:497-502 (1990)).
Vaccinia virus recombinant-based vaccines which express the F or G envelope glycoprotein have also been explored. These recombinants express RSV glycoproteins which are indistinguishable from the authentic viral counterpart, and rodents infected intradermally with vaccinia-RSV F and G recombinants developed high levels of specific antibodies that neutralized viral infectivity. Indeed, infection of cotton rats with vaccinia-F recombinants stimulated almost complete resistance to replication of RSV in the lower respiratory tract and significant resistance in the upper tract. Olmsted et al., Proc. Natl. Acad. Sci. USA 83:7462-7466 (1986). However, immunization of chimpanzees with vaccinia-F and -G recombinant provided almost no protection against RSV challenge in the upper respiratory tract (Collins et al., Vaccine 8:164-168 (1990)) and inconsistent protection in the lower respiratory tract (Crowe et al., Vaccine 11:1395-1404 (1993).
The unfulfilled promises of attenuated RSV strains, subunit vaccines, and other strategies for RSV vaccine development underscores a need for new methods to develop novel RSV vaccines, particularly methods for manipulating recombinant RSV to incorporate genetic changes to yield new phenotypic properties in viable, attenuated RSV recombinants. However, manipulation of the genomic RNA of RSV and other negative-sense RNA viruses has heretofore proven difficult. Major obstacles in this regard include non-infectivity of naked genomic RNA of these viruses, poor viral growth in tissue culture, lengthy replication cycles, virion instability, a complex genome, and a refractory organization of gene products.
Recombinant DNA technology has made it possible to recover infectious negative-stranded RNA viruses from cDNA, to genetically manipulate viral clones to construct novel vaccine candidates, and to rapidly evaluate their level of attenuation and phenotypic stability (for reviews, see Conzelmann, J. Gen. Virol. 77:381-89 (1996); Palese et al., Proc. Natl. Acad. Sci. U.S.A. 93:11354-58, (1996)). In this context, recombinant rescue has been reported for infectious respiratory syncytial virus (RSV), parainfluenza virus (PIV), rabies virus (RaV), vesicular stomatitis virus (VSV), measles virus (MeV), and Sendai virus (SeV) from cDNA-encoded antigenomic RNA in the presence of essential viral proteins (see, e.g., Garcin et al., EMBO J. 14:6087-6094 (1995); Lawson et al., Proc. Natl. Acad. Sci. U.S.A. 92:4477-81 (1995); Radecke et al., EMBO J. 14:5773-5784 (1995); Schnell et al., EMBO J. 13:4195-203 (1994); Whelan et al., Proc. Natl. Acad. Sci. U.S.A. 92:8388-92 (1995); Hoffman et al., J. Virol. 71:4272-4277 (1997); Kato et al., Genes to Cells 1:569-579 (1996), Roberts et al., Virology 247(1), 1-6 (1998); Baron et al., J. Virol. 71:1265-1271 (1997); International Publication No. WO 97/06270; Collins et al., Proc. Natl. Acad. Sci. USA 92:11563-11567 (1995); Durbin et al., Virology 235:323-332 (1997); U.S. patent application Ser. No. 08/892,403, filed Jul. 15, 1997 (corresponding to published International Application No. WO 98/02530 and priority U.S. Provisional Application Nos. 60/047,634, filed May 23, 1997, 60/046,141, filed May 9, 1997, and 60/021,773, filed Jul. 15, 1996); Juhasz et al., J. Virol. 71(8):5814-5819 (1997); He et al. Virology 237:249-260 (1997); Baron et al. J. Virol. 71:1265-1271 (1997); Whitehead et al., Virology 247(2):232-9 (1998a); Whitehead et al., J. Virol. 72(5):4467-4471 (1998b); Jin et al. Virology 251:206-214 (1998); Bucholz et al. J. Virol. 73:251-259 (1999); and Whitehead et al., J. Virol. 73:(4)3438-3442 (1999), each incorporated herein by reference).
Among the remaining challenges to RSV vaccine development is the difficulty of achieving vaccine candidates that are effective against a broad range of existing and emergent strains and subgroups of RSV. In particular, it will be useful to provide a RSV subgroup B-specific vaccine virus, as well as multivalent vaccines to provide protection against both RSV A and RSV B subgroups. In this context, recent research has focused on development of chimeric viruses to carry antigenic determinants between viral strains. For example, the HN and F glycoproteins of human parainfluenza virus type 3 (PIV3) have been replaced by those of human parainfluenza virus type 1 (HPIV1), and the resulting chimeric virus grew in cell culture and in experimental animals with an efficiency similar to its wild-type parents (Tao et al., J. Virol. 72(4):2955-61 (1998), incorporated herein by reference. Also reported is a chimeric measles virus where the H and F glycoproteins were replaced with the G glycoprotein of vesicular stomatitis virus, which was inserted with or without replacement of the cytoplasmic and transmembrane region of G with that of measles virus F (Spielhofer et al., J. Virol. 72(3):2150-9 (1998)). This yielded a chimeric virus that was reportedly 50-fold reduced in growth. In a third example, Jin et al., Virology 251(1):206-14 (1998) report a subgroup A virus which expresses the G protein of a subgroup B RSV as an additional gene (Jin et al., Virology 251(1):206-14 (1998)). However, since the F protein also exhibits significant subgroup-specificity, it would be preferable to express both subgroup B glycoproteins in a subgroup B-specific vaccine. In addition, production of a chimeric A-B virus will not produce a viable vaccine candidate without further modifications to achieve proper attenuation and virulence.
Accordingly, an urgent need remains in the art for tools and methods to engineer safe and effective vaccines to alleviate the serious health problems attributable to RSV, particularly that will be effective against multiple existing and emergent strains and subgroups of RSV. Quite surprisingly, the present invention satisfies these and other related needs.